Viscosity index improver, process for preparing the same and lubricating oil composition

ABSTRACT

A viscosity index improver for addition to lubricating oil or the like, which comprises a graft copolymer having structural units of an oil-soluble polymer (a) comprising an olefin polymer and structural units of a copolymer (b) of a monomer having a peroxide bond with a (meth)acrylate or the like or a copolymer (c) of a monomer having a peroxide bond with 2-hydroxyethyl(meth)acrylate or the like. The improver is excellent in the effect of improving the viscosity index, the thickening effect, the fluidity at low temperatures and the shear stability, and has no fear of causing phase separation in lubricating oil.

TECHNICAL FIELD

The present invention relates to a viscosity index improver to be addedto lubricating oils such as engine oils, gear oils, transmission oilsand hydraulic oils, a process for producing the same and a lubricatingoil composition. More particularly, the present invention relates to aviscosity index improver which has excellent viscosity index improvingproperty, thickening property, low-temperature fluidity and shearstability and which does not undergo separation in a lubricating oil, toa process for producing the same and to a lubricating oil compositionprepared by incorporating the viscosity index improver into alubricating oil.

BACKGROUND ART

It is practically preferred that lubricating oils employable as engineoils, hydraulic oils, etc. undergo least viscosity change over a widetemperature range from low temperatures to high temperatures. Viscosityindex is employed as a measure of this property, and the greater theviscosity index is, the higher is the stability to temperature changes.It is known that viscosity index of an oil can be improved by adding toit a certain kind of polymer. As such polymers, for example,polymethacrylates (PMA) (Japanese Unexamined Patent Publication No. Hei7-62372), olefin copolymers (OCP) (Japanese Patent Publication No. Sho46-34508), hydrogenated styrene/diene copolymers (SDC) (Japanese PatentPublication No. Sho 48-39203) and polyisobutylenes (PIB) are employed.SDCs assuming polymeric forms of block copolymer (Japanese UnexaminedPatent Publication No. Sho 49-47041) and star-shaped polymer (JapaneseUnexamined Patent Publication No. Sho 52-96695) have been developed inaddition to SDCs assuming a form of random copolymer.

Lubricating oils incorporated with these polymers exhibitcharacteristics of their own. More specifically, although PMAs haveexcellent viscosity index improving properties and also have pourpoint-lowering actions, they have poor thickening effects. In order toimprove the thickening effects, the molecular weight of PMAs may beincreased. However, PMAs having increased molecular weights come to haveextremely low stability to shear forces to be caused by stirring etc. oflubricating oils. PIBs have high thickening effects but poor viscosityindex improving properties. OCPs and SDCs have high thickening effectsand have low viscosity at low temperatures, but their viscosity indeximproving properties are inferior to those of PMAs. Meanwhile, thosePMAs which are copolymerized with polar monomers can impart tolubricating oils detergent dispersing performance of dispersing sludgein the lubricating oils easily compared with other improvers (JapanesePatent Publication No. Sho 51-20273 and Japanese Unexamined PatentPublication No. Hei 5-222389).

While multi-grade oils having excellent viscosity index improvingperformances are now generally used as lubricating oils, viscosity indeximprovers having higher performance are now in demand so as to complywith recent requirements including reduction of energy consumption. Itcan be contemplated to use a mixture of PMA and OCP or SDC as acomposition which satisfies such requirements. However, these compoundshave poor compatibility with each other, so that if a mere mixture ofsuch compounds is added to a lubricating oil, the lubricating oilseparates into two phases. Therefore, in order to avoid such separation,there are proposed graft copolymers consisting of two different kinds ofpolymers (Japanese Patent Publication Nos. Sho 59-40194, Sho 62-16997and Hei 4-50328; Japanese Unexamined Patent Publication No. Hei6-346078, etc.).

However, the processes for producing such graft copolymers each employ aconventional radical polymerization initiator to carry out graftcopolymerization of a methacrylate monomer in the presence of anoil-soluble olefin copolymer. The graft copolymerization in this casetakes place based on olefin copolymer chain radicals to be formed whenthe radical polymerization initiator extracts hydrogen atoms from theolefin copolymer. However, a homopolymer of the methacrylate monomer isalso formed in a large amount when the graft copolymerization is carriedout to bring about a low graft efficiency.

Accordingly, such graft copolymers involve a problem in that they havepoor properties as viscosity index improvers including thickeningeffect, viscosity behavior, etc. Further, in those graft copolymershaving extremely low graft efficiency values, lubricating oilcompositions incorporated with them undergo separation. Accordingly, inorder to comply with all conceivable severer requirements for lubricantsin the future, development of high-performance viscosity index improversis in demand.

The present invention was accomplished in view of such problems inherentin the prior art, and it is an objective of the invention to provide aviscosity index improver which has excellent viscosity index improvingproperty, excellent thickening effect, excellent low-temperaturefluidity and excellent shear stability and which is free from the fearof separation in a lubricating oil, as well as, a process for producingthe same and a lubricating oil composition containing the same.

DISCLOSURE OF THE INVENTION

The viscosity index improver according to the present invention containsa graft copolymer which has a structural unit formed by an oil-solublepolymer (a) containing an olefin polymer and a structural unit formed bya peroxy bond-containing polymer (b) or a polymer (c) containing aperoxy bond and a predetermined functional group.

The oil-soluble polymer (a), the polymers (b) and (c) are preferably asfollows:

(a) one or more oil-soluble polymers selected from the group consistingof ethylene/α-olefin copolymers, styrene/hydrogenated diene copolymers,hydrogenated polybutadienes, hydrogenated polyisoprenes, polybutenes,ethylene/(meth)acrylic acid ester copolymers and ethylene/vinyl estercopolymers;

(b) a copolymer to be obtained by copolymerizing a peroxybond-containing monomer with one or more monomers selected from thegroup consisting of (meth)acrylic acid esters, unsaturated dicarboxylicacid esters, vinyl esters of fatty acids having 2 to 18 carbon atoms andaromatic monomers having 8 to 12 carbon atoms; and

(c) a copolymer to be obtained by copolymerizing a peroxybond-containing monomer with one or more monomers selected from thegroup consisting of (meth)acrylic acid esters, unsaturated dicarboxylicacid esters, vinyl esters of fatty acids having 2 to 18 carbon atoms andaromatic monomers having 8 to 12 carbon atoms and with one or moremonomers selected from the group consisting of tertiarynitrogen-containing monomers and monomers containing either or both of ahydroxyl group and an ether bond.

It should be noted here that in the present invention, acrylate andmethacrylate are generally referred to as (meth)acrylate; while allyland methallyl are generally referred to as (meth)allyl.

Further, the viscosity index improver according to the present inventioncontains the following graft copolymer (i) and either or both of thefollowing copolymer (ii) and the following copolymer (iii):

(i) a graft copolymer formed by the oil-soluble polymer (a) and theperoxy bond-containing copolymer (b) or (c);

(ii) the oil-soluble polymer (a); and

(iii) a copolymer to be derived from the copolymer (b) or (c).

Incidentally, the copolymer to be derived from the copolymer (b) or (c)means a copolymer which is not grafted to or with the oil-solublepolymer (a).

Meanwhile, the process for producing a viscosity index improveraccording to the present invention comprises heating a mixture of theoil-soluble polymer (a) and a peroxy bond-containing copolymer (b) or(c) to such a temperature that the peroxy bond cleaves to effectgrafting.

Further, the process for producing a viscosity index improver accordingto the present invention comprises adding to a lubricant base oil thefollowing monomer mixture (d) or (e) and a radical polymerizationinitiator to effect copolymerization of the monomer mixture at such atemperature that the radical polymerization initiator decomposes andthat the peroxy bond of the peroxy bond-containing monomer does notsubstantially cleave, and adding the oil-soluble polymer (a) to theresulting mixture to be dissolved therein, followed by heating of themixture to such a temperature that peroxy bonds derived from the peroxybond-containing monomer cleave to effect grafting:

(d) a monomer mixture containing a peroxy bond-containing monomer andone or more monomers selected from the group consisting of (meth)acrylicacid esters, unsaturated dicarboxylic acid esters, vinyl esters of fattyacids having 2 to 18 carbon atoms and aromatic monomers having 8 to 12carbon atoms; or

(e) a monomer mixture containing a peroxy bond-containing monomer, oneor more monomers selected from the group consisting of (meth)acrylicacid esters, unsaturated dicarboxylic acid esters, vinyl esters of fattyacids having 2 to 18 carbon atoms and aromatic monomers having 8 to 12carbon atoms and one or more monomers selected from the group consistingof tertiary nitrogen-containing monomers and monomers containing eitheror both of a hydroxyl group and an ether bond.

Further, the process for producing a viscosity index improver comprisesadding to a lubricant base oil the oil-soluble monomer (a), the monomermixture (d) or (e) and a radical polymerization initiator to effectcopolymerization of the monomer mixture at such a temperature that theradical polymerization initiator decomposes and that the peroxy bond ofthe peroxy bond-containing monomer does not substantially cleave,followed by heating of the resulting mixture to such a temperature thatperoxy bonds derived from the peroxy bond-containing monomer cleave toeffect grafting.

Further, the process for producing a viscosity index improver comprisesa first step of adding to an aqueous suspension of particles of theoil-soluble polymer (a) the monomer mixture (d) or (e) and a radicalpolymerization initiator, and heating the resulting mixture at such atemperature that the radical polymerization initiator does notsubstantially decompose to impregnate the particulate oil-solublepolymer (a) with the monomer mixture and the radical polymerizationinitiator, followed by heating of the resulting aqueous suspension atsuch a temperature that the radical polymerization initiator decomposesand that the peroxy bond of the peroxy bond-containing monomer does notsubstantially cleave to effect copolymerization of the monomer mixturein the particulate oil-soluble polymer (a) and obtain a graft precursor;and a second step of melt-kneading the graft precursor at such atemperature that peroxy bonds derived from the peroxy bond-containingmonomer cleave to effect grafting.

In addition, the lubricating oil composition according to the presentinvention is prepared by adding to a lubricating oil the viscosity indeximprover described above.

Embodiments of the present invention will be described below in detail.

First, the viscosity index improver according to the present inventionwill be described.

The viscosity index improver contains a graft copolymer in which astructural unit formed by an oil-soluble polymer (a) containing anolefin polymer is grafted to or with a structural unit formed by aperoxy bond-containing polymer (b) or a polymer (c) containing a peroxybond and a certain functional group. The oil-soluble polymer (a) hasthickening performance, while the polymer (b) or (c) has pour pointlowering performance and viscosity index improving performance. Thegraft copolymer containing the structural unit formed by the polymer (a)and the structural unit formed by the polymer (b) or (c) exhibitperformances of the respective polymers synergistically and also hasshear stability.

Incidentally, viscosity index can be calculated, for example, inaccordance with JIS K-2283, and the greater the value is, the smaller isthe temperature-dependent viscosity change and the more preferred.

In the graft copolymer, the structural unit formed by the oil-solublepolymer (a) constitutes a backbone chain, and the structural unit formedby the polymer (b) or (c) constitutes side chains or vice versa.

Shear stability is supposed to be improved because the viscosity indeximprover is a graft copolymer and if the side chains of the graftcopolymer are severed, the backbone chain remains intact to hardlyundergo viscosity reduction. Further, since the viscosity index improveris formed by the structural unit formed by an oil-soluble polymer (a)containing an olefin polymer and the structural unit formed by thepolymer (b) or (c) containing, for example, a poly(meth)acrylate polymerhaving poor compatibility which are chemically bound to each other inthe form of graft copolymer, it also has a characteristic that it doesnot cause phase separation in a lubricating oil.

It is an essential characteristic feature of the viscosity indeximprover that it contains a peroxy bond-containing polymer. That is,polymer radicals to be formed by cleavage of the peroxy bond by heating,electron transfer or other methods are considered to be bound with theoil-soluble polymer with high efficiency to form a graft copolymer, andthus the graft copolymer can exhibit excellent performance as aviscosity index improver. This is the point which is quite differentfrom the technique in which a graft copolymer of an olefin polymer and apoly(meth)acrylate polymer to be prepared by using a conventionalradical polymerization initiator is employed as a viscosity indeximprover.

The oil-soluble polymer (a) contains one or more polymers selected fromthe group consisting of ethylene/α-olefin copolymers,styrene/hydrogenated diene copolymers, hydrogenated polybutadienes,hydrogenated polyisoprenes, polybutenes, ethylene/(meth)acrylic acidester copolymers and ethylene/vinyl ester copolymers. These polymers aresubstantially oil-soluble and preferably each have a weight averagemolecular weight of 5000 to 1000000. If the weight average molecularweight is smaller than 5000, the resulting viscosity index improvershows a low thickening effect, whereas if it is greater than 1000000,the resulting viscosity index improver shows low shear stability.Further, the polymer (a) desirably shows oil solubility particularly inlubricating oils.

The α-olefin moieties of the ethylene/α-olefin copolymers preferablyinclude those having 3 to 20 carbon atoms per monomer molecule, sincethey are readily available. Particularly, propylene and 1-butene aremost preferred, since they are inexpensive and are easily available.While the content of ethylene in the graft copolymer may notparticularly be limited, it is preferably 20 to 80% by weight in view oflow-temperature fluidity.

The styrene/hydrogenated diene copolymers are copolymers of styrene withdiene compounds such as butadiene and isoprene, in which the unsaturatedbond in the diene moiety of each polymer is substantially hydrogenated.Typically, they include random copolymers, block copolymers, star-shapedcopolymers, etc. While the styrene content in the copolymer may notparticularly be limited, it is preferably 70% by weight or less, sinceif it is added in an excessive amount, solubility of the resultingcopolymer in a lubricating oil is lowered.

The block copolymers can be exemplified by those described in JapaneseUnexamined Patent Publication No. Sho 49-47401 and Hei 1-149899 in whichpolystyrene segments (S) and hydrogenated polydiene segments (D) arebound to each other alternately and include, for example, S-D di-blockcopolymers, S-D-S and D-S-D tri-block copolymers. Meanwhile, thestar-shaped copolymers can be exemplified by those having a hydrogenatedpolymer chain of diene or a styrene polymer chain on the benzene nucleusas described in Japanese Unexamined Patent Publication No. Sho 52-96695and Hei 7-268047. The polybutenes include, for example, poly(1-butene)and polyisobutylene.

The ethylene/(meth)acrylic acid ester copolymers are random copolymersof ethylene and (meth)acrylic acid esters as described, for example, inJapanese Unexamined Patent Publication No. Hei 7-268373. The(meth)acrylic acid esters preferably include C₁ -C₂₂alkyl(meth)acrylates and typically, for example, methyl(meth)acrylate,ethyl(meth)acrylate, butyl(meth)acrylate, lauryl(meth)acrylate andstearyl(meth)acrylate. The ethylene/vinyl ester copolymers are randomcopolymers of ethylene and vinyl esters as described, for example, inJapanese Unexamined Patent Publication No. Sho 64-48892, and they mayfurther be copolymerized with third monomers. The vinyl esterspreferably include those of fatty acids having 2 to 18 carbon atoms andtypically vinyl acetate, vinyl propionate, etc.

Ethylene/α-olefin copolymers or styrene/hydrogenated diene copolymersare preferred among other oil-soluble polymers (a) because of theirexcellent thickening performance.

Next, the peroxy bond-containing copolymer (b) is a copolymer to beformed by copolymerizing a peroxy bond-containing monomer (component α)with one or more monomers (component β) selected from the groupconsisting of (meth)acrylic acid esters, unsaturated dicarboxylic acidesters, vinyl esters of fatty acids having 2 to 18 carbon atoms andaromatic monomers having 8 to 12 carbon atoms.

Referring to the amount of each component in this peroxy bond-containingcopolymer (b), the component a and the component β are preferably 0.05to 50% by weight and 50 to 99.95% by weight respectively. If thecomponent a is more than 50% by weight, the resulting viscosity indeximprover comes to have an extremely great molecular weight to exhibitpoor shear stability, and there is a liability of forming a gel contentwhich is insoluble in lubricating oils. Meanwhile, if the component α isless than 0.05% by weight, a low graft efficiency is brought about to beliable to cause separation of lubricating oils into two phases.

The peroxy bond-containing copolymer (c) is a copolymer to be obtainedby copolymerizing a monomer constituting the peroxy bond-containingcopolymer (b) with one or more monomers (component γ) selected from thegroup consisting of tertiary nitrogen-containing monomers and monomerscontaining either or both of a hydroxyl group and an ether bond.

Referring to the amount of each component in this peroxy bond-containingcopolymer (c), the component α and the component β are preferably 0.05to 50% by weight and 50 to 99.95% by weight respectively, based on thesame reasons as in the case of the peroxy bond-containing copolymer (b).The amount of component γ is preferably 20% by weight or less, since theperformance as the viscosity index improver is deteriorated if it isused in an excessive amount, and the resulting improver becomesinsoluble in lubricating oils.

The peroxy bond of the peroxy bond-containing copolymer (b) or (c) isusually on the side chain. (Meth)acrylic acid esters are preferred amongothers as the component β because of their excellent performance asviscosity index improvers, particularly their low-temperature fluidity.The copolymers (b) and (c) may further be copolymerized with othermonomers copolymerizable with them.

If the weight average molecular weight of the peroxy bond-containingcopolymer (b) or (c) is too great, the resulting viscosity indeximprover comes to have poor shear stability; whereas it if it is toosmall, the improver exhibits poor thickening effect. Accordingly, theweight average molecular weight is preferably 5000 to 1000000, morepreferably 10000 to 500000.

As the peroxy bond-containing monomer, any of known peroxybond-containing monomers can be employed, and preferably monomersrepresented by the following general formulae (1) to (3) can beemployed. These monomers may be used singly or in the form of mixture oftwo or more of them.

General formula (1) ##STR1## wherein R₁ represents a hydrogen atom or amethyl group; R₂ represents a hydrogen atom or a methyl group; R₃ and R₄each represent an alkyl group having 1 to 4 carbon atoms; R₅ representsan alkyl group having 1 to 12 carbon atoms, a cycloalkyl group having 3to 12 carbon atoms, a phenyl group or an alkyl-substituted phenyl group;and n is 1 to 5;

General formula (2) ##STR2## wherein R₆ represents a hydrogen atom or amethyl group; R₇ represents a hydrogen atom or a methyl group; R₈ and R₉each represent an alkyl group having 1 to 4 carbon atoms; R₁₀ representsan alkyl group having 1 to 12 carbon atoms, a cycloalkyl group having 3to 12 carbon atoms, a phenyl group or an alkyl-substituted phenyl group;and n is 0 to 4;

General formula (3) ##STR3## wherein R₁₁ represents a hydrogen atom or amethyl group; and R₁₂ and R₁₃ each represent an alkyl group having 1 to4 carbon atoms; R₁₄ represents an alkyl group having 1 to 12 carbonatoms, a cycloalkyl group having 3 to 12 carbon atoms, a phenyl group oran alkyl-substituted phenyl group.

Peroxy bond-containing monomers represented by the general formula (1)include typically t-butyl peroxy(meth)acryloyloxyethylcarbonate, t-butylperoxy(meth)acryloyloxyethoxyethylcarbonate, t-butylperoxy(meth)acryloyloxyisopropylcarbonate, t-amylperoxy(meth)acryloyloxyethylcarbonate, t-amylperoxy(meth)acryloyloxyisopropylcarbonate, t-hexylperoxy(meth)acryloyloxyethylcarbonate, t-hexylperoxy(meth)acryloyloxyisopropylcarbonate, t-octylperoxy(meth)acryloyloxyethylcarbonate, cumylperoxy(meth)acryloyloxyethylcarbonate, p-isopropylcumylperoxy(meth)acryloyloxyethylcarbonate, p-menthylperoxy(meth)acryloyloxyethylcarbonate, 1-cyclohexyl-1-methylethylperoxy(meth)acryloyloxyethylcarbonate, etc.

Peroxy bond-containing monomers represented by the general formula (2)include typically t-butyl peroxy(meth)allylcarbonate, t-butylperoxy(meth)allyloxyethylcarbonate, t-butylperoxy(meth)allyloxyethoxyethylcarbonate, t-amylperoxy(meth)allylcarbonate, t-hexyl peroxy(meth)allylcarbonate, t-octylperoxy(meth)allylcarbonate, cumyl(meth)allyl carbonate, etc.

Peroxy bond-containing monomers represented by the general formula (3)include typically t-butyl peroxymethylfumarate, t-butylperoxyethylfumarate, t-butyl peroxy-n-propylfumarate, t-butylperoxyisopropylfumarate, t-butyl peroxy-n-buthylfumarate, t-butylperoxy-t-butylfumarate, t-butyl peroxy-n-octylfumarate, t-butylperoxy-2-ethylhexylfumarate, t-butyl peroxyphenylfumarate, t-butylperoxy-m-toluylfumarate, t-butyl peroxycyclohexylfumarate, t-amylperoxy-n-propylfumarate, t-amyl peroxyisopropylfumarate, t-amylperoxy-n-butylfumarate, t-amyl peroxyphenylfumarate, t-hexylperoxyethylfumarate, t-hexyl peroxyisoproylfumarate, t-hexylperoxy-t-butylfumarate, t-hexyl peroxy-2-ethylhexylfumarate, t-octylperoxymethylfumarate, t-octyl peroxyisopropylfumarate, t-octylperoxy-n-octylfumarate, t-octyl peroxycyclohexylfumarate, cumylperoxyisopropylfumarate, p-menthyl peroxyisopropylfumarate, etc.

Of these peroxy bond-containing monomers, preferred monomers are t-butylperoxyacryloyloxyethylcarbonate, t-butylperoxymethacryloyloxyethylcarbonate, t-butyl peroxyallylcarbonate,t-butyl peroxymethallylcarbonate and t-butyl peroxyisopropylfumarate.These peroxy bond-containing monomers have heat decompositiontemperatures of 80° C. or higher and are easily available andeconomical.

Next, the (meth)acrylic acid esters are preferably those having C₁ -C₂₂alkyl moieties and include, for example, methyl(meth)acrylate,ethyl(meth)acrylate, n-propyl(meth)acrylate, isopropyl(meth)acrylate,n-butyl(meth)acrylate, isobutyl(meth)acrylate, t-butyl(meth)acrylate,hexyl(meth)acrylate, cyclohexyl(meth)acrylate, heptyl(meth)acrylate,octyl(meth)acrylate, 2-ethylhexyl(meth)acrylate, decyl(meth)acrylate,isodecyl(meth)acrylate, lauryl(meth)acrylate, tridecyl(meth)acrylate,myristyl(meth)acrylate, pentadecyl(meth)acrylate, cetyl(meth)acrylate,stearyl(meth)acrylate, eicosyl(meth)acrylate and behenyl(meth)acrylate.

While these (meth)acrylic acid esters may be used singly, a mixture of 1to 40% by weight of (meth)acrylates having C₁ -C₆ alkyl groups, 30 to90% by weight of (meth)acrylates having C₇ -C₁₅ alkyl groups and 1 to40% by weight of (meth)acrylates having C₁₆ -C₂₂ alkyl groups ispreferably used. The reason is that use of the thus combined mixture canimprove low-temperature fluidity and viscosity index of the viscosityindex improver and also solubility thereof in lubricating oils.

The unsaturated dicarboxylic acid esters are preferably those having C₁-C₂₂ alkyl groups. Such esters include, for example, dimethyl maleate,dibutyl maleate, dihexyl fumarate, dioctadecyl fumarate and dilaurylitaconate.

The vinyl esters of fatty acids having 2 to 18 carbon atoms include, forexample, vinyl acetate, vinyl propionate, vinyl 2-ethylhexanoate, vinyldecanoate and vinyl dodecanoate.

The aromatic monomers having 8 to 12 carbon atoms include, for example,styrene, α-methylstyrene, α-ethylstyrene, vinyltoluene, dimethylstyrene,t-butylstyrene, chlorostyrene and bromostyrene.

The tertiary nitrogen-containing monomers include, for example,vinylpyrrolidone, vinylpyridine, vinylimidazol,dimethylaminoethyl(meth)acrylate, dimethylaminopropyl(meth)acrylate,diethylaminoethyl(meth)acrylate, dimethylaminoethyl(meth)acrylamide,dimethyl(meth)acrylamide and (meth)acrylonitrile.

The hydroxyl- or ether bond-containing monomers are hydroxyl- or etherbond-containing alkyl(meth)acrylates represented by the followinggeneral formula (4): ##STR4## wherein R₁₅ represents a hydrogen atom ora methyl group; and R₁₆ represents a C₁ -C₆ alkyl group having at leastone hydroxyl group or a group of the following formula (5): ##STR5##(wherein R₁₇ and R₁₈ each represent a hydrogen atom or a methyl group;R₁₉ represents a hydrogen atom or a C₁ -C₃ alkyl group; and n is aninteger of 1 to 60).

Such alkyl(meth)acrylates include hydroxyalkyl(meth)acrylates such as2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate,1-methyl-2-hydroxyethyl(meth)acrylate, 2-hydroxybutyl(meth)acrylate andglycerol mono(meth)acrylate; polyethylene glycol mono(meth)acrylate,polypropylene glycol mono(meth)acrylate, methoxypolyethylene glycolmono(meth)acrylate, octyloxypolypropylene glycol mono(meth)acrylate,etc.

These tertiary nitrogen-containing monomers and the hydroxyl- or etherbond-containing monomers have detergent dispersing performance, i.e. anaction of maintaining sludge particles suspended in lubricating oils.

While the weight ratio of the structural unit derived from theoil-soluble polymer (a) to the structural unit derived from the peroxybond-containing copolymer (b) or (c) in the viscosity index improver maynot particularly be limited, it is preferably in the range of 5/95 to95/5 in view of overall performance of lubricating oils. In this case,the more the units derived from the oil-soluble polymer (a), the greaterthe thickening effect; whereas the more the units derived from theperoxy bond-containing copolymer (b) or (c), the greater the viscosityindex.

Meanwhile, the weight average molecular weight of the polymer ispreferably 10000 to 2000000, particularly 20000 to 500000, in view ofthickening effect and shear stability.

Further, the viscosity index improver may be a mixture containing alinear polymer derived from the oil-soluble polymer (a), copolymer (b)or copolymer (c) in addition to the graft copolymer composed of theoil-soluble polymer (a) and the peroxy bond-containing copolymer (b) or(c). The mixture may contain such linear polymer which is by-produced inthe process of producing the improver or which is added afterward.Viscosity index improvers containing such linear polymers exhibit thedesired thickening effects, viscosity index improving effects, shearstabilizing effects, etc. based on the graft copolymers or based on thesynergistic effects to be brought about the graft copolymers and thelinear polymer or on the effects to be brought about by the linearpolymers, respectively.

The amount of linear polymer in the polymer mixture may not particularlybe limited. However, if the amount of the linear polymer is too much,excellent performances of the graft polymer are impaired, and theresulting lubricating oil composition containing it is liable to beseparated into two phases. Accordingly, the linear polymer is addedpreferably in an amount of 90% by weight or less, particularly in anamount of 50% by weight or less.

Next, a process for producing the viscosity index improver will bedescribed.

The process for producing a viscosity index improver comprises heating amixture of the oil-soluble polymer (a) and the peroxy bond-containingcopolymer (b) or (c) to such a temperature that the peroxy bond cleavesto effect grafting.

Further, the process for producing a viscosity index improver comprisesa first step of polymerizing the monomer mixture (d) or (e) to form aperoxy bond-containing copolymer (b) or (c) and a second step ofgrafting a mixture of the oil-soluble polymer (a) and the peroxybond-containing copolymer (b) or (c).

As a method for forming the peroxy bond-containing copolymer (b) or (c)in the first step, there may be mentioned the conventional radicalpolymerization technique employing a radical polymerization initiator.The method of polymerization may be any of bulk polymerization, solutionpolymerization, suspension polymerization and emulsion polymerization,and preferably solution polymerization or suspension polymerization isemployed. This polymerization may be carried out in the presence of theoil-soluble polymer (a).

The solution employable in the solution polymerization may notparticularly be limited and can be selected from various kinds ofsolvents such as saturated hydrocarbons, aromatic hydrocarbons, ketones,fatty acid esters and carbonic acid esters, as well as, lubricant baseoils such as mineral oils and synthetic oils. Among these solvents, thelubricant base oils such as mineral oils are preferred, because theresulting solutions can be as such used as lubricating oil compositions.

In the suspension polymerization, it may be carried out employing anoil-soluble polymer (a) impregnated with a radical polymerizationinitiator and the monomer mixture (d) or (e). In this case, theoil-soluble polymer (a) preferably assumes a form of powder or pellethaving a particle size of about 0.1 to 10 mm. The impregnation treatmentis preferably carried out at a highest possible temperature. However, ifthe treatment is carried out at an extremely high temperature, theamount of unimpregnated copolymer is formed in a large amount to lowerthe graft efficiency in the second step. Accordingly, the impregnationtreatment is carried out generally at a temperature at least SoC lowerthan the half-life (10 hours) temperature of the radical polymerizationinitiator such that the impregnation rate may be 50% by weight or more.

Suspension polymerization employs water as a medium and an ordinarydispersant, an emulsifier, etc. While the aqueous suspension may has adesired concentration, it is generally prepared by adding to waterreaction components in an amount of 5 to 150 parts by weight of per 100parts by weight of water.

The monomer mixture (d) or (e) may be of such a composition and in suchan amount that it can form a peroxy bond-containing copolymer (b) or(c), respectively. Typically, the monomer mixture (d) or (e) is the samecomposition of monomers as that described with respect to the copolymer(b) or (c).

The polymerization can be carried out using a radical polymerizationinitiator. While the radical polymerization initiator may notparticularly be limited, it is preferred to employ a polymerizationinitiator having a half-life (10 hours) temperature of 100° C. or lower.Typical polymerization initiators include, for example, t-butylperoxypivalate, t-hexyl peroxypivalate, t-butyl peroxyneodecanoate,t-octyl peroxyneodecanoate, t-butyl peroxy-2-ethylhexanoate, t-amylperoxy-2-ethylhexanoate, t-hexyl peroxy-2-ethylhexanoate, t-octylperoxy-2-ethylhexanoate, t-butyl peroxyisobutyrate, lauroyl peroxide,benzoyl peroxide, 3,5,5-trimethylhexanoyl peroxide, diisobutyrylperoxide, diisopropyl peroxidicarbonate, di-2-ethylhexylperoxidicarbonate, 1,1-bis(t-butylperoxy)cyclohexane,1,1-bis(t-butylperoxy)3,3,5-trimethylcyclohexane and2,2'-azobisisobutyronitrile. While the usage of radical polymerizationinitiator should be selected such that the polymer to be formed may havea desired molecular weight, usually it is preferably 0.01 to 5% byweight relative to the monomers.

The polymerization temperature and the polymerization time should beselected such that the peroxy bond of the peroxy bond-containing monomerdoes not cleave, and polymerization is preferably carried out at atemperature of 50 to 80° C. for 3 to 10 hours.

The weight average molecular weight of the peroxy bond-containingcopolymer (b) or (c) to be prepared in the first step is preferablyadjusted to be within the range of 5000 to 1000000, preferably in therange of 10000 to 500000, as described above. Further, a chain transferagent may be used in the polymerization so as to achieve molecularweight adjustment.

The method for grafting the mixture of the oil-soluble polymer (a) andthe peroxy bond-containing copolymer (b) or (c) in the second stepincludes preferably heating of the mixture in a solvent such as amineral oil or melt kneading of the mixture with heating in the absenceof solvent etc.. The oil-soluble polymer (a) may be added in the firststep or at the beginning of the second step.

The grafting is carried out at such a temperature that peroxy bondsderived from the peroxy bond-containing copolymer (b) or peroxybond-containing monomer in (c) cleave, and the temperature is preferably80 to 300° C., more preferably 100 to 200° C.

Melt kneading methods include those employing the conventional kneaderssuch as a Banbury mixer, a pressure kneader, a Ko-kneader, adouble-screw extruder and a mixing roll.

In the second step, while the weight ratio of the oil-soluble polymer(a) to the peroxy bond-containing copolymer (b) or (c) may notparticularly be limited, it is preferably in the range of 5/95 to 95/5.Meanwhile, the weight average molecular weight of the polymer to beobtained is adjusted preferably to be within the range of 10000 to2000000, more preferably within the range of 20000 to 500000.

Typical process for producing the viscosity index improver preferablyincludes the following three:

The first process will be described below. To a lubricant base oil areadded the monomer mixture (d) or (e) and a radical polymerizationinitiator to effect copolymerization of the monomer mixture at such atemperature that the radical polymerization initiator decomposes andthat the peroxy bond of the peroxy bond-containing monomer does notsubstantially cleave, and then the oil-soluble polymer (a) is added tothe resulting mixture to be dissolved therein, followed by heating ofthe mixture to such a temperature that peroxy bonds derived from theperoxy bond-containing monomer cleave to effect grafting.

The second process will be described below. To a lubricant base oil areadded the oil-soluble polymer (a), the monomer mixture (d) or (e) and aradical polymerization initiator to effect copolymerization of themonomer mixture at such a temperature that the radical polymerizationinitiator decomposes and that the peroxy bond of the peroxybond-containing monomer does not substantially cleave, followed byheating of the mixture to such a temperature that the peroxy bond of theperoxy bond-containing monomer cleaves to effect grafting.

The third process will be described below. To an aqueous suspension ofparticles of the oil-soluble polymer (a) are added the monomer mixture(d) or (e) and a radical polymerization initiator, and the resultingmixture is heated under the condition where decomposition of the radicalpolymerization initiator substantially does not occur to impregnate theparticulate oil-soluble polymer (a) with the monomer mixture and theradical polymerization initiator.

Subsequently, the resulting aqueous suspension is heated to such atemperature that the radical polymerization initiator decomposes andthat the peroxy bond of the peroxy bond-containing monomer does notsubstantially cleave to effect copolymerization of the monomer mixturein the particulate oil-soluble polymer (a) and obtain a graft precursor(first step). This graft precursor is melt-kneaded at such a temperaturethat the peroxy bond cleaves to effect grafting (second step).

The first and second processes enjoy merits in that they give graftcopolymers with high efficiency, and that viscosity index improvers canbe obtained in the form of solution, eliminating the procedures ofdissolving the polymers in lubricating oils. Meanwhile, the thirdprocess enjoys merits in that a graft copolymer with high graftefficiency can be obtained, and that a high-purity polymer can beobtained, leading to reduction in the transportation cost. The polymerto be obtained according to the third process may as necessary bedissolved in a solvent such as a mineral oil to be prepared into a formof lubricating oil composition.

Further, the viscosity index improver may be synthesized to have amolecular weight larger than the value to be expected in the final useand then subjected to mechanical or thermal molecular weight reductionby conventional procedures in the art to adjust the molecular weight tobe within the desired range.

After production of the viscosity index improver, the improver mayfurther be grafted, as necessary, with a monomer having detergentdispersing performance by conventional procedures.

The amount of graft copolymer to be contained in the viscosity indeximprover can be measured according to the ordinary fractionalprecipitation and is expressed in terms of graft efficiency. Graftefficiency of the copolymer (b) or (c) to the oil-soluble polymer (a) ispreferably 20% or more, more preferably 50% or more.

The amount of graft copolymer can also be expressed by the number ofbranch chains (branch number) per molecule of the polymer to be measuredusing a gel permeation chromatograph (GPC-LALLS) equipped with a lightscattering detector. The greater branch number means the greater degreeof grafting. The branch number is preferably 1 or more.

Next, the lubricating oil composition will be described.

While the lubricating oil composition is prepared by incorporating theviscosity index improver into a lubricant base oil, the composition maycontain other components such as lubricating oil additives and the likewhich are generally added to lubricating oils.

Such other components include, for example, other known viscosity indeximprovers including oily agents such as long-chain fatty acids; abrasionpreventives such as phosphoric acid esters and metal dithiophosphates;extreme pressure additives such as organic sulfurous compounds andorganic molybdenum compounds; rust preventives such as carboxylic acids,sulfonic acid salts and phosphoric acid salts; detergents such as metalsalts including sulfonates, phenates and phosphonates; dispersants suchas succinimide; pour point depressants such as poly(meth)acrylate andcondensates of chlorinated paraffin with naphthalene or phenol;antioxidants such as zinc thiophosphate, amines and phenols; andpoly(meth)acrylates.

Referring to lubricating oil compositions containing the viscosity indeximprovers at high concentration, they are used as component lubricatingoil additives if they contain the viscosity index improvers only or aspackage lubricating oil additives if they contain various kinds ofadditives. A lubricating oil composition containing various kinds ofadditives adjusted to desired concentrations respectively are used aslubricating oils such as gasoline engine oils, diesel engine oils, gearoils, transmission oils, hydraulic oils, power steering oils andshock-absorbing oils.

The lubricant base oil can be exemplified by the conventional mineraloils to be obtained by purifying crude oils, for example, paraffinic andnaphthenic neutral oils, hydrocarbon series synthetic lubricating oils,ester series synthetic lubricating oils, MLDW oils and high-viscosityindex mineral oils containing paraffin isomers, or mixtures of theseoils, and preferably neutral oils.

The amount of viscosity index improver in the lubricating oilcomposition, in terms of concentrate, is in such a range that thecomposition can be handled with ease, typically in an amount of 10 to60% by weight. The amount of viscosity index improver actually used in alubricating oil is adjusted such that the oil may have a desired gradeof viscosity, typically in the range of 0.5 to 20% by weight, forexample, 2 to 4% by weight.

As described above, the viscosity index improvers and the processes forproducing the same in the above embodiments enjoy the following merits:

(1) The viscosity index improvers have excellent viscosity indeximproving performances and thickening effects, since they each contain agraft copolymer consisting of the structural unit formed by the specificoil-soluble polymer (a) and the structural unit formed by the copolymer(b) or (c), so that a high-performance lubricating oil can be obtainedby adding the viscosity index improver in a small amount to alubricating oil;

(2) The polymers assume a structure of graft copolymer, and if the sidechains of the graft copolymer are severed, the backbone chain remainsintact to maintain viscosity at a predetermined level. Accordingly, theviscosity index improvers show excellent shear stability when subjectedto high shearing actions;

(3) Since the respective polymers are chemically bound to one another toassume a graft structure, the viscosity index improvers do not undergophase separation in lubricating oils to show stability over extendedperiods;

(4) The viscosity index improvers show, based mainly on the peroxybond-containing copolymer (b) and (c), excellent pour point depressingperformance or excellent low-temperature fluidity;

(5) The viscosity index improvers can impart to lubricating oils, basedon the peroxy bond-containing copolymer (c), excellent detergentdispersing performance of dispersing sludge particles in the lubricatingoils; and

(6) According to the process for producing the viscosity index improverof the present invention, the above-described viscosity index improverscontaining the graft copolymers can be produced easily and with highgraft efficiency values, respectively.

BEST MODE FOR CARRYING OUT THE INVENTION

Next, the present invention will be described by way of nonlimitativeexamples. It should be noted here that % in the following descriptionand tables means all % by weight. Further, the molecular weight meansthe weight average molecular weight (Mw) determined by means of gelpermeation chromatography (GPC) using tetrahydrofuran as a developingsolvent.

Incidentally, abbreviations used in the following description and tablesare as shown below:

    ______________________________________                                        MEC:      t-butyl peroxymethacryloyloxyethylcarbonate                         AC:       t-butyl peroxyallylcarbonate                                        IPF:      t-butyl peroxyisopropylfumarate                                     MMA:      methyl methacrylate                                                 BMA:      butyl methacrylate                                                  IDMA:     isodecyl methacrylate                                               LMA:      lauryl methacrylate                                                 CMA:      cetyl methacrylate                                                  SMA:      stearyl methacrylate                                                EMA:      eicosyl methacrylate                                                ST:       styrene                                                             DMF:      dimethyl fumarate                                                   DBF:      dibutyl fumarate                                                    DDF:      didodecyl fumarate                                                  DMM:      dimethyl maleate                                                    DBM:      dibutyl maleate                                                     DDM:      didodecyl maleate                                                   VAc:      vinyl acetate                                                       VD:       vinyl dodecanoate                                                   NVP:      N-vinyl-2-pyrrolildone                                              DMAEMA:   dimethylaminoethyl methacrylate                                     HEMA:     hydroxyethyl methacrylate                                           PE350:    polyethylene glycol monomethacrylate (trade name:                             BLEMMER PE-350, Nippon Oil & Fats Co., Ltd.)                        THPO:     3,5,5-trimethyhexanoyl peroxide                                     LPO:      lauroyl peroxide                                                    BPO:      benzoyl peroxide                                                    NDM:      n-dodecylmercaptan                                                  SDC1:     [polystyrene-hydrogenated polyisoprene] diblock                               copolymer (styrene content: 35%; molecular                                    weight: 130000)                                                     SDC2:     [hydrogenated polyisoprene-polystyrene-                                       hydrogenated polyisoprene] triblock                                           copolymer (styrene content: 30%; molecular weight:                            170000)                                                             SDC3:     [polystyrene-hydrogenated polyisoprene-                                       polystyrene] triblock copolymer (styrene content:                             40%; molecular weight: 110000)                                      SDC4:     [polystyrene-hydrogenated polybutadiene-                                      polystyrene] triblock copolymer (styrene content:                             20%; molecular weight: 90000)                                       OCP1:     ethylene/propylene random copolymer                                           (ethylene content: 55%; molecular weight: 80000)                    OCP2:     ethylene/1-butene random copolymer                                            (ethylene content: 47%; molecular weight: 130000)                   HPB:      hydrogenated polybutadiene (molecular weight:                                 95000)                                                              HPI:      hydrogenated polyisoprene (molecular weight:                                  85000)                                                              SPI:      star-shaped copolymer having 8 hydrogenated                                   polyisoprene chains on an average bound to a                                  polydivinyl benzene nucleus (hydrogenated                                     polyisoprene chain molecular weight: 85000)                         PIB:      polyisobutylene (molecular weight: 53000)                           EDMA:     ethylene/dodecyl methacrylate copolymer                                       (ethylene content: 55%; molecular weight:                                     105000)                                                             EVD:      ethylene/vinyl dodecanoate copolymer                                          (ethylene content: 60%; molecular weight:                                     140000)                                                             ______________________________________                                    

EXAMPLE 1

To a 1-liter reaction vessel equipped with a stirrer, a dropping funnel,a thermometer, a purge gas inlet and a water-cooled condenser wascharged 150 g of 100 neutral oil (Super Oil AL, Nippon Oil Co., Ltd.,prepared by purifying final petroleum distillate), and after the oil washeated to 70° C. under nitrogen gas blowing, a mixture of 2 g of MEC, 10g of MMA, 61 g of IDMA, 15 g of CMA, 8 g of SMA, 4 g of EMA and 1 g ofTHPO was added dropwise thereto with stirring over 2 hours. Aftercompletion of dropping, stirring was continued as such for 6 hours tocomplete polymerization and give a peroxy bond-containing copolymer. Theconcentration of the polymer in the solution was 40%, and the weightaverage molecular weight (Mw) of the polymer was 149000.

Next, a mixture of 110 g of an oil-soluble polymer SDC dissolved in 15 gof 100 neutral oil was added to this solution to effect reaction withstirring at 120° C. for 6 hours under nitrogen gas blowing to give ahomogeneous, transparent and viscous graft copolymer solution. Theweight average molecular weight of the polymer was 238000, and theconcentration of the polymer in the solution was 40%. The branch numberper molecule of the polymer also measured by using GPC-LALLS (Toso) wasfound to be 1.6. Further, the polymer was isolated by means of rubbermembrane dialysis employing a petroleum ether as an extraction solventand dissolved in toluene, and the resulting solution was then subjectedto fractional precipitation by adding methyl ethyl ketone thereto slowlyto find that the content of the graft copolymer in the polymer was 36%.

When the 100 neutral oil solution containing 40% of the polymer thusobtained was maintained at 100° C. so as to examine its stability, thesolution underwent no phase separation even after 100 days.

To the polymer solution obtained was added 100 neutral oil (kineticviscosity at 100° C.: 4.4 cSt; viscosity index: 101) to adjust thekinetic viscosity at 100° C. to be about 11.5 cSt. The polymer contentof the solution was 2.9%. The polymer content represents the thickeningeffect, and the smaller the polymer content is, the higher is thethickening effect. Further, the following tests were carried outemploying this solution. The results are summarized in Table 1.

Kinetic Viscosity and Viscosity Index

Kinetic viscosity was measured at 40° C. and 100° C. in accordance withJIS K-2283 to calculate viscosity index based on the thus measuredvalues. The greater the viscosity index is, the smaller is thetemperature-dependent viscosity change and the higher is the stability.

Pour Point

Pour point was measured in accordance with JIS K-2269. The lower thepour point is, the better is the fluidity of the lubricating oil at lowtemperatures.

Shear Stability

Shear stability was measured at 100° C. in accordance with ASTM D-3945to calculate shear stability index (SSI) using the following expression:

    SSI=[(Vu-Vs)/(Vu-Vo)]×100,

wherein Vu represents viscosity before shearing; V_(s) representsviscosity after shearing; and Vo represents viscosity of the base oil.

The smaller the shear stability index is, the better the shear stabilityof the lubricating oil.

EXAMPLES 2 TO 20

Graft copolymers were synthesized in the same manner as in Example 1except that the monomer composition of the peroxy bond-containingcopolymer (B) and the amount of the oil soluble polymer (A) were changedas shown in Tables 1 and 2. In the same manner as in Example 1, thegraft copolymers were subjected to the stability test, and further asolution having a kinetic viscosity at 100° C. of about 11.5 cSt wasprepared by adding 100 neutral oil to each polymer solution andsubjected to the performance test. The results of polymerization and ofthe tests are shown in Tables 1 to 4.

                  TABLE 1                                                         ______________________________________                                        Example      1      2       3     4     5                                     ______________________________________                                        Copolymer B                                                                   Loading (g)                                                                   Monomer                                                                       MEC          2      1       4     30    5                                     MMA          10     5       16    --    10                                    BMA          --     15      --    20    15                                    IDMA         61     --      40    --    --                                    LMA          --     60      10    35    40                                    CMA          15     12      10    --    12                                    SMA          8      7       20    10    6                                     EMA          4      --      --    5     2                                     NVP          --     --      --    --    5                                     DMAEMA       --     --      --    --    5                                     Initiator                                                                     THPO         1      1       2     --    1.5                                   LPO          --     --      --    0.5   --                                    BPO          --     --      --    --    --                                    Base oil     150    150     150   150   150                                   Solid content (%)                                                                          40     40      40    40    40                                    Mw × 10.sup.-4                                                                       14.9   15.5    9.8   23.3  14.1                                  Polymer A                                                                     Loading (g)                                                                   SDC1         10     --      --    --    20                                    SDC2         --     10      --    --    --                                    OCP1         --     --      10    --    --                                    OCP2         --     --      --    20    --                                    Base oil     15     15      15    30    30                                    Copolymer formed                                                              Concentration (%)                                                                          40     40      40    40    40                                    Mw × 10.sup.-4                                                                       23.8   25.4    16.4  33.3  21.9                                  Stability (day)                                                                            >100   >100    >100  >100  >100                                  Polymer content (%)                                                                        2.9    2.7     3.4   2.2   2.5                                   Viscosity index                                                                            231    225     222   219   212                                   Pour point (° C.)                                                                   -40    -42.5   -40   -35   -42.5                                 SSI          29     28      25    32    31                                    ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Example      6      7       8     9     10                                    ______________________________________                                        Copolymer B                                                                   Loading (g)                                                                   Monomer                                                                       MEC          5      5       5     --    --                                    IPF          --     --      --    5     10                                    MMA          10     10      5     10    10                                    BMA          --     15      10    10    --                                    IDMA         55     --      --    --    55                                    LMA          --     40      40    40    --                                    CMA          12     15      15    15    11                                    SMA          6      --      --    --    6                                     EMA          2      10      10    10    3                                     ST           --     --      10    --    --                                    DBF          --     --      --    5     --                                    NVP          10     --      --    5     5                                     DMAEMA       --     5       5     --    --                                    Initiator                                                                     THPO         1      --      1.3   --    1                                     LPO          --     1       --    2     --                                    BPO          --     --      --    --    --                                    Base oil     150    150     150   150   150                                   Solid content (%)                                                                          40     40      40    40    40                                    Mw × 10.sup.-4                                                                       16.8   17.4    13.5  11.4  16.1                                  Polymer A                                                                     Loading (g)                                                                   HPB          15     --      --    --    --                                    HPI          --     15      --    --    --                                    PIB          --     --      10    --    --                                    EDMA         --     --      --    20    --                                    EVD          --     --      --    --    20                                    Base oil     22.5   22.5    15    30    30                                    Copolymer formed                                                              Concentration (%)                                                                          40     40      40    40    40                                    Mw × 10.sup.-4                                                                       25.6   26.7    19.5  19.7  20.4                                  Stability (day)                                                                            >100   >100    >100  >100  >100                                  Polymer content (%)                                                                        2.7    2.7     3.1   2.6   2.7                                   Viscosity index                                                                            231    228     211   216   210                                   Pour point (° C.)                                                                   -35    -40     -40   -32.5.                                                                              -32.5                                 SSI          29     30      31    27    26                                    ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Example      11     12      13    14    15                                    ______________________________________                                        Copolymer B                                                                   Loading (g)                                                                   Monomer                                                                       MEC          2      4       --    --    --                                    AC           --     --      5     3     5                                     MMA          8      9       --    --    --                                    BMA          --     --      10    10    --                                    IDMA         33     59      30    30    --                                    LMA          18     --      20    20    --                                    CMA          20     13      14    12    --                                    SMA          9      5       11    5     --                                    EMA          --     2       --    --    --                                    ST           --     --      5     --    --                                    DBF          --     --      --    10    --                                    DMM          --     --      --    --    5                                     DBM          --     --      --    --    5                                     DDM          --     --      --    --    10                                    VAc          --     --      --    --    35                                    VD           --     --      --    --    35                                    NVP          --     --      --    --    5                                     HEMA         --     5       5     10    --                                    PE350        10     3       --    --    --                                    Initiator                                                                     THPO         1      1       0.7   2     2                                     Base oil     150    150     100   100   100                                   Solid content (%)                                                                          40     40      50    50    50                                    Mw × 10.sup.-4                                                                       18.3   17.6    25.1  18.4  18.6                                  Polymer A                                                                     Loading (g)                                                                   SDC1         --     --      50    --    --                                    SDC2         --     --      --    5     --                                    SDC4         --     10      --    --    --                                    OCP1         --     --      --    --    10                                    SPI          10     --      --    --    --                                    Base oil     15     15      50    5     10                                    Copolymer formed                                                              Concentration (%)                                                                          40     40      50    50    60                                    Mw × 10.sup.-4                                                                       35.3   24.4    17.3  28.2  29.1                                  Stability (day)                                                                            >100   >100    >100  >100  >100                                  Polymer content (%)                                                                        2.8    2.8     2.1   3.1   2.8                                   Viscosity index                                                                            224    229     192   231   227                                   Pour point (° C.)                                                                   -40    -30     -40   -40   -35                                   SSI          32     29      19    30    27                                    ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        Example      16     16      17    18    20                                    ______________________________________                                        Copolymer B                                                                   Loading (g)                                                                   Monomer                                                                       MEC          --     2       3     1     5                                     AC           --     --      4     --    --                                    IPF          3      --      --    3     --                                    MMA          --     --      6     --    3                                     BMA          --     --      --    10    3                                     IDMA         --     --      57    49    50                                    CMA          --     --      10    15    15                                    SMA          10     6       7     8     8                                     EMA          7      4       3     4     4                                     ST           --     --      --    --    4                                     DMF          10     10      --    --    --                                    DBM          10     20      --    --    3                                     DDF          45     48      --    --    --                                    NVP          --     5       5     --    --                                    DMAEMA       15     5       5     --    --                                    HEMA         --     --      --    2     5                                     PE350        --     --      --    8     --                                    Initiator                                                                     LPO          1      1       1     2     2                                     Base oil     100    150     150   67    67                                    Solid content (%)                                                                          50     40      40    60    60                                    Mw × 10.sup.-4                                                                       22.9   17.7    16.5  24.4  23.5                                  Polymer A                                                                     Loading (g)                                                                   SDC1         --     20      --    10    --                                    SDC2         --     20      --    --    10                                    SDC3         --     --      20    --    --                                    OCP2         --     --      20    --    --                                    HPB          --     --      --    10    --                                    HPI          10     --      --    --    10                                    Base oil     10     60      60    13    13                                    Copolymer formed                                                              Concentration (%)                                                                          60     40      40    60    60                                    Mw × 10.sup.-4                                                                       29.6   18.1    19.5  23.3  22.1                                  Stability (day)                                                                            >100   >100    >100  >100  >100                                  Polymer content (%)                                                                        2.7    1.9     1.8   2.3   2.4                                   Viscosity index                                                                            219    196     194   225   223                                   Pour point (° C.)                                                                   -32.5  -40     -40   -40   -40                                   SSI          28     20      18    25    24                                    ______________________________________                                    

EXAMPLE 21

To a 1-liter vessel equipped with a stirrer, a dropping funnel, athermometer, a purge gas inlet and a water-cooled condenser were charged36 g of 100 neutral oil, 43 g of SDC1 as the oil-soluble polymer, 2 g ofMEC, 10 g of MMA, 61 g of IDMA, 15 g of CMA, 8 g of SMA and 4 g of EMA,and the resulting mixture was heated to 70° C. with stirring undernitrogen gas blowing to prepare a solution, to which a mixture of 2 g ofTHPO and 178.5 g of 100 neutral oil was added dropwise over 2 hours.After completion of dropping, stirring was continued as such for 6 hoursto complete polymerization and give a 40% solution of the mixture of theperoxy bond-containing copolymer and the oil-soluble polymer.

Further, this solution was allowed to react at 120° C. with stirringunder nitrogen gas blowing for 6 hours to give a homogeneous,transparent and viscous graft copolymer solution. The concentration ofthe polymer in the solution was 40%, and the weight average molecularweight of the polymer was 136000. The branch number per molecule of thepolymer measured by using GPC-LALLS was found to be 2.2. Further, thecontent of the graft copolymer in the polymer measured by fractionalprecipitation was 48%.

In the same manner as in Example 1, the stability test was carried outusing a 100 neutral oil solution containing 40% of the polymer obtained,and further a solution having a kinetic viscosity at 100° C. of about11.5 cSt was prepared by adding 100 neutral oil to the polymer solutionand subjected to the performance test. The results are summarized inTable 5.

EXAMPLES 22 TO 30

Graft copolymers were synthesized in the same manner as in Example 21except that the monomer composition of the copolymer (B), theoil-soluble polymer (A) and loading were changed as shown in Table 3,and tests were carried out in the same manner as in Example 21. Resultsof polymerization and of the tests are shown in Tables 5 and 6.

                  TABLE 5                                                         ______________________________________                                        Example      21     22      23    24    25                                    ______________________________________                                        Copolymer B                                                                   Loading (g)                                                                   Monomer                                                                       MEC          2      3       4     30    --                                    IPF          --     --      --    --    5                                     MMA          10     10      6     --    --                                    BMA          --     --      --    20    --                                    IDMA         61     60      40    --    --                                    LMA          --     --      10    15    --                                    CMA          15     15      10    --    1 0                                   SMA          8      8       20    10    --                                    EMA          4      4       --    5     5                                     ST           --     --      10    20    --                                    DMF          --     --      --    --    10                                    DBM          --     --      --    --    10                                    DDF          --     --      --    --    50                                    NVP          --     --      --    --    5                                     DMAEMA       --     --      --    --    5                                     Initiator                                                                     THPO         2      2       2     --    1.5                                   LPO          --     --      --    0.5   --                                    BPO          --     --      --    --    --                                    Polymer A                                                                     Loading (g)                                                                   SDC1         43     --      --    --    --                                    SDC2         --     --      --    --    --                                    SDC3         --     --      25    --    --                                    SDC4         --     --      --    --    67                                    OCP1         --     43      --    --    --                                    OCP2         --     --      --    25    --                                    Base oil     214.5  214.5   187.5 187.5 250                                   Copolymer formed                                                              Concentration (%)                                                                          40     40      40    40    40                                    Mw × 10.sup.-4                                                                       19.8   18.4    22.4  23.3  16.9                                  Stability (day)                                                                            >100   >100    >100  >100  >100                                  Polymer content (%)                                                                        2.9    2.7     3.4   2.2   2.5                                   Viscosity index                                                                            211    205     222   219   198                                   Pour point (° C.)                                                                   -42.5  -37.5   -35   -35   -30                                   SSI          19     21      25    27    14                                    ______________________________________                                    

                  TABLE 6                                                         ______________________________________                                        Example      26     27      28    29    30                                    ______________________________________                                        Copolymer B                                                                   Loading (g)                                                                   Monomer                                                                       MEC          --     3       2     --    10                                    AC           5      --      --    5     --                                    MMA          --     10      5     10    10                                    BMA          --     15      10    10    --                                    IDMA         --     --      --    --    55                                    LMA          --     42      43    40    --                                    CMA          --     15      15    15    11                                    SMA          --     --      --    --    6                                     EMA          --     10      10    10    3                                     ST           --     --      10    --    --                                    DMM          5      --      --    --    --                                    DBM          5      --      --    --    --                                    DDM          10     --      --    --    --                                    VAc          30     --      --    --    --                                    VD           35     --      --    --    --                                    NVP          10     --      --    --    --                                    DMAEMA       --     5       5     --    --                                    HEMA         --     --      --    10    --                                    PE350        --     --      --    --    5                                     Initiator                                                                     THPO         1      --      1.3   --    1                                     LPO          --     1       --    2     --                                    BPO          --     --      --    --    --                                    Polymer A                                                                     Loading (g)                                                                   HPB          11     --      --    --    --                                    HPI          --     15      --    --    --                                    PIB          --     --      10    --    --                                    EDMA         --     --      --    20    --                                    EVD          --     --      --    --    20                                    Base oil     167    115     257   180   180                                   Copolymer formed                                                              Concentration (%)                                                                          40     50      30    40    40                                    Mw × 10.sup.-4                                                                       28.6   24.7    20.5  18.9  19.1                                  Stability (day)                                                                            >100   >100    >100  >100  >100                                  Polymer content (%)                                                                        2.7    2.7     3.1   2.9   2.8                                   Viscosity index                                                                            208    221     210   219   221                                   Pour point (° C.)                                                                   -30    -40     -37.5 -40   -40                                   SSI          29     27      27    25    26                                    ______________________________________                                    

EXAMPLE 31

To a 2-liter-capacity reaction vessel equipped with a stirrer, adropping funnel, a thermometer, a purge gas inlet and a water-cooledcondenser was charged 550 g of water, and further 0.6 g of polyvinylalcohol was dissolved therein as a suspending agent, followed byaddition of 100 g of particulate SDC1 (particle diameter: 1 to 2 mm) wasadded as the oil-soluble polymer to be dispersed therein with stirring.To the resulting dispersion were added 1 g of MEC, 10 g of MMA, 62 g ofIDMA, 15 g of CMA, 8 g of SMA, 4 g of EMA, 0.5 g of n-dodecylmercaptanand 1 g of THPO, and the resulting mixture was stirred. The mixture wasthen heated to 60° C. under nitrogen gas blowing to effect impregnationof the oil-soluble polymer with the radical polymerization initiator andthe vinyl monomer with stirring for one hour. Subsequently, the mixturewas heated to 70° C., and stirring was continued at the same temperaturefor 6 hours to complete polymerization and give a graft precursor inwhich the peroxy bond-containing copolymer is present in the oil-solublepolymer.

The graft precursor was filtered out, washed with water, dried and thensubjected to kneading over a Banbury mixer (Toyo Seiki Seisakusho) at140° C. at 100 rpm for one hour to obtain a graft copolymer having anaverage molecular weight of 130000. The branch number per molecule ofthe polymer measured by using GPC-LALLS was found to be 3.1. Further,the content of the graft copolymer in the polymer measured by fractionalprecipitation was 68%.

When 10 g of the polymer thus obtained was added to 90 g of 100 neutraloil and the resulting mixture was heated, the polymer dissolvedcompletely in the oil to give a homogeneous, transparent and viscousliquid.

In the same manner as in Example 1, the resulting solution was subjectedto the stability test, and further a solution having a kinetic viscosityat 100° C. of about 11.5 cSt was prepared by adding 100 neutral oil tothe polymer solution and subjected to the performance test. The resultsare summarized in Table 7.

EXAMPLES 32 TO 40

Graft copolymers were synthesized in the same manner as in Example 31except that the monomer composition of the copolymer (B), theoil-soluble polymer (A) and loading were changed as shown in Table 4,and tests were carried out in the same manner as in Example 31. Resultsof polymerization and of the tests are shown in Tables 7 and 8.

                  TABLE 7                                                         ______________________________________                                        Example      31     32      33    34    35                                    ______________________________________                                        Copolymer B                                                                   Loading (g)                                                                   Monomer                                                                       MEC          1      2       --    --    --                                    AC           --     --      2     10    --                                    IPF          --     --      --    --    2                                     MMA          10     5       18    --    --                                    BMA          --     14      --    10    --                                    IDMA         62     --      40    10    --                                    LMA          --     60      10    45    --                                    CMA          15     12      10    10    8                                     SMA          8      7       20    10    --                                    EMA          4      --      --    5     5                                     ST           --     --      --    --    10                                    DMF          --     --      --    --    10                                    DBF          --     --      --    --    10                                    DDF          --     --      --    --    50                                    NVP          --     --      --    --    5                                     Initiator                                                                     THPO         1      --      --    2     2                                     LPO          --     --      --    --    --                                    BPO          --     2       1     --    3                                     NDM          0.5    0.5     0.3   0.7   --                                    Polymer A                                                                     Loading (g)                                                                   SDC1         100    --      --    --    --                                    SDC2         --     --      --    --    --                                    SDC3         --     100     --    --    --                                    SDC4         --     --      --    --    50                                    OCP1         --     --      300   --    --                                    OCP2         --     --      --    --    --                                    SPI          --     --      --    100   --                                    Copolymer formed                                                              Mw × 10.sup.-4                                                                       13.0   15.8    8.8   14.1  9.6                                   Stability (day)                                                                            >100   >100    >100  >100  >100                                  Polymer content (%)                                                                        2.2    1.5     1.9   2.7   2.9                                   Viscosity index                                                                            188    182     178   189   193                                   Pour point (° C.)                                                                   -40    -35     -40   -30   -32.5                                 SSI          14     21      16    18    25                                    ______________________________________                                    

                  TABLE 8                                                         ______________________________________                                        Example      36     37      38    39    40                                    ______________________________________                                        Copolymer B                                                                   Loading (g)                                                                   Monomer                                                                       MEC          3      2       3     2     --                                    AC           --     --      --    --    5                                     IPF          --     --      --    1     --                                    MMA          5      10      6     --    --                                    BMA          5      --      --    10    --                                    IDMA         23     --      57    50    --                                    LMA          15     49      --    --    --                                    CMA          19     15      10    15    --                                    SMA          10     8       7     8     --                                    EMA          --     4       3     4     --                                    DBF          10     --      --    --    --                                    DMM          --     --      --    --    5                                     DBM          --     --      --    --    5                                     DDM          --     --      --    --    10                                    VAc          --     --      --    --    35                                    VD           --     --      --    --    35                                    NVP          --     5       5     --    --                                    DMAEMA       --     5       5     --    --                                    HEMA         6      --      --    2     5                                     PE350        4      --      --    8     --                                    Initiator                                                                     THPO         --     --      5     --    --                                    LPO          --     --      --    2     2                                     BPO          4      4       --    --    --                                    NDM          --     --      --    0.4   0.4                                   Polymer A                                                                     Loading (g)                                                                   SDC1         --     20      --    --    --                                    SDC2         100    20      --    --    --                                    OCP1         --     --      20    --    --                                    OCP2         --     --      20    --    --                                    HPB          --     --      --    50    --                                    HPI          --     --      --    50    --                                    SPI          --     --      --    --    --                                    PIB          --     --      --    100   --                                    EDMA         --     --      --    --    50                                    EVD          --     --      --    --    50                                    Copolymer formed                                                              Mw × 10.sup.-4                                                                       10.2   7.5     8.6   6.7   13.2                                  Stability (day)                                                                            >100   >100    >100  >100  >100                                  Polymer content (%)                                                                        2.6    2.3     2.6   3.1   2.8                                   Viscosity index                                                                            185    201     204   194   184                                   Pour point (° C.)                                                                   -40    -37.5   -40   -40   -35                                   SSI          15     19      21    16    17                                    ______________________________________                                    

Comparative Example 1

To a 1-liter reaction vessel equipped with a stirrer, a dropping funnel,a thermometer, a purge gas inlet and a water-cooled condenser wascharged 150 g of 100 neutral oil, and after the oil was heated to 70° C.under nitrogen gas blowing, a mixture of 10 g of MMA, 63 g of IDMA, 15 gof CMA, 8 g of SMA, 4 g of EMA and 1 g of THPO was added dropwisethereto with stirring over 2 hours. After completion of dropping,stirring was continued as such for 6 hours to complete polymerizationand give a copolymer. The concentration of the polymer in the solutionwas 40%, and the weight average molecular weight (Mw) of the polymer was158000.

In the same manner as in Example 1, the resulting polymer solution wassubjected to the stability test, and further a solution having a kineticviscosity at 100° C. of about 11.5 cSt was prepared by adding 100neutral oil to the polymer solution and subjected to the performancetest. The results are shown in Table 9.

Comparative Example 2

To 100 g of the solution containing 40% of the polymer obtained inComparative Example 1 were added 8 g of SDC1, 12 g of 100 neutral oiland 1 g of t-butyl peroxybenzoate, and the resulting mixture was reactedat 120° C. under nitrogen gas blowing for 6 hours. The polymer had aweight average molecular weight Mw of 102000, and the branch number permolecule of the polymer was 0.3. Further, the content of the graftcopolymer in the polymer was 5%.

The polymer solution obtained was subjected to the performance test inthe same manner as in Example 1. The results are shown in Table 9.

Comparative Example 3

To a 1-liter reaction vessel equipped with a stirrer, a dropping funnel,a thermometer, a purge gas inlet and a water-cooled condenser werecharged 180 g of 100 neutral oil and 20 g of OCP1 to prepare a solution,and after the solution was heated to 70° C. under nitrogen gas blowing,a mixture of 10 g of MMA, 61 g of IDMA, 15 g of CMA, 8 g of SMA and 4 gof EMA and 1 g of THPO was added dropwise thereto over 2 hours. Aftercompletion of dropping, stirring was continued as such for 6 hours,followed by addition of 0.5 g of t-butyl peroxybenzoate thereto. Theresulting mixture was then heated to 130° C., and stirring was continuedfor 6 hours with addition of 0.5 g of t-butyl peroxygenzoate after 1hour and 3 hours in the meantime to give a copolymer. The concentrationof the polymer in the solution was 40%, and the weight average molecularweight of the polymer was 178000. The branch number per molecule of thepolymer was 0.8. Further, the content of the graft copolymer in thepolymer was 12%.

The polymer solution obtained was subjected to the performance test inthe same manner as in Example 1. The results are shown in Table 9.

Comparative Examples 4 to 7

The performance test described in Example 1 was carried out using SDC 1(Comparative Example 4), OCP1 (Comparative Example 5), HPB (ComparativeExample 6) or PIB (Comparative Example 7) singly. The results are shownin Table 9.

                  TABLE 9                                                         ______________________________________                                        Comparative Example                                                                       1      2      3    4    5    6    7                               ______________________________________                                        Stability (day)                                                                           >100    20    >100 >100 >100 >100 >100                            Polymer content (%)                                                                       4.8    4.1    3.8  2.1  2.2  2.7  2.6                             Viscosity index                                                                            221   205     195  166  163  162  158                            Pour point (° C.)                                                                   -40   -35     -40  -15  -15  -15  -15                            SSI          42     30     30   18   19   17   26                             ______________________________________                                    

EXAMPLES 41 AND 42

The polymer solutions obtained in Examples 5 and 36 were diluted with100 neutral oil so that they may have a polymer concentration of 3%, andthe resulting solutions were subjected to the test stipulated in JISK-2514 for 72 hours to measure the sludge amounts, respectively, to findthat the sludge amounts in the solutions were 0.2% and 0.3%respectively.

Comparative Examples 8 and 9

The polymer solution obtained in Comparative Example 1 and SDC1 werediluted with 100 neutral oil so that they may have a polymerconcentration of 3% respectively. The resulting solutions were subjectedto sludge quantitative determination as described in Example 41, to findthat the sludge amounts in the solutions were 4.5% and 5.2%respectively.

As is clear from the results shown in Tables 1 to 8, the viscosity indeximprovers of Examples 1 to 40 are excellent in stability in solutions,viscosity index improving property, thickening effect, low-temperaturefluidity and shear stability. In addition, the viscosity improver ofExample 1 showed a great thickening effect and excellent shear stabilitycompared with that of Comparative Example 1 containing onlypolymethacrylates of the same composition as in Example 1.

Further, as shown in Table 9, the viscosity index improvers of Exampleshave excellent stability in solutions and excellent thickening effectscompared with those of Comparative Examples 2 and 3 which are preparedby carrying out grafting of polymer mixtures using the ordinary radicalpolymerization initiators respectively. This is because that theviscosity index improvers of Examples contain large amounts of graftcopolymers compared with those prepared by the conventional methods.

Further, the viscosity index improvers of Examples each have excellentviscosity index and low-temperature fluidity compared with those ofComparative Examples 4 to 7 which contain only oil-soluble polymers.

In addition, it can be understood that the viscosity index improvers ofExamples 41 and 42 can be easily imparted with detergent dispersingperformance.

It should be noted here that the present invention is not to be limitedto Examples given above but may be modified arbitrarily withoutdeparting from the gist of the invention.

INDUSTRIAL APPLICABILITY

As has been described heretofore, the viscosity index improversaccording to the present invention have excellent viscosity indeximproving properties, thickening effects, low-temperature fluidity andshear stability, and high-performance lubricating oils can be obtainedby adding them in small amounts to lubricating oils such as engine oilsand gear oils, so that they can be utilized suitably as lubricating oiladditives.

What is claimed is:
 1. A viscosity index improver comprising a graftcopolymer having a molecular weight in the range of between about 10,000to 2,000,000 as the weight average; said graft copolymer having astructural unit formed by an oil-soluble polymer (a) containing anolefin polymer and a structural unit formed by a peroxy bond-containingpolymer (b) obtained from a monomer represented by the formulas (1), (2)or (3) as follows: ##STR6## wherein R₁ represents a hydrogen atom or amethyl group; R₂ represents a hydrogen atom or a methyl group; R₃ and R₄each represent an alkyl group having 1 to 4 carbon atoms; R₅ representsan alkyl group having 1 to 12 carbon atoms, a cycloalkyl group having 3to 12 carbon atoms, a phenyl group or an alkyl-substituted phenyl group;and n is 1 to 5; ##STR7## wherein R₆ represents a hydrogen atom or amethyl group; R₇ represents a hydrogen atom or a methyl group; R₈ and R₉each represent an alkyl group having 1 to 4 carbon atoms; R₁₀ representsan alkyl group having 1 to 12 carbon atoms, a cycloalkyl group having 3to 12 carbon atoms, a phenyl group or an alkyl-substituted phenyl group;and n is 0 to 4; ##STR8## wherein R₁₁ represents a hydrogen atom or amethyl group; R₁₂ and R₁₃ each represent an alkyl group having 1 to 4carbon atoms; R₁₄ represents an alkyl group having 1 to 12 carbon atoms,a cycloalkyl group having 3 to 12 carbon atoms, a phenyl group or analkyl-substituted phenyl group;or a polymer (c) obtained bycopolymerizing the peroxy bond-containing copolymer (b) with a monomerbeing selected from the group consisting of the tertiarynitrogen-containing monomers and monomers containing either or both of ahydroxyl group and an ether bond, said tertiary nitrogen-containingmonomers being selected from the group consisting of vinylpyrrolidone,vinylpyridine, vinylimidazol, dimethylaminoethyl(meth)acrylate,dimethylaminopropyl(meth)acrylate, dimethylaminoethyl(meth)acrylamide,dimethyl(meth)acrylamide and (meth)acrylonitrile, said hydroxyl or etherbond containing monomers are alkyl(meth)acrylates of the formula:##STR9## wherein R₁₅ is a hydrogen atom or a methyl group, R₁₆ is a C₁-C₆ alkyl group having at least one hydroxyl group or a group of thefollowing formula: ##STR10## wherein R₁₇ and R₁₈ each represent ahydrogen atom or a methyl group, R₁₉ is a hydrogen atom or a C₁ -C₃alkyl group, and n is an integer of 1 to 60; the weight ratio of theoil-soluble polymer (a) to the peroxy bond containing polymer (b) or (c)being in the range of 5 to 95 to 95 to
 5. 2. The viscosity indeximprover according to claim 1, wherein the oil-soluble polymer (a), thepolymers (b) and (c) are as follows:(a) one or more oil-soluble polymersselected from the group consisting of ethylene/α-olefin copolymers,styrene/hydrogenated diene copolymers, hydrogenated polybutadienes,hydrogenated polyisoprenes, polybutenes, ethylene/(meth)acrylic acidester copolymers and ethylene/vinyl ester copolymers; (b) a copolymer tobe obtained by copolymerizing a peroxy bond-containing monomer with oneor more monomers selected from the group consisting of (meth)acrylicacid esters, unsaturated dicarboxylic acid esters, vinyl esters of fattyacids having 2 to 18 carbon atoms and aromatic monomers having 8 to 12carbon atoms; and (c) a copolymer to be obtained by copolymerizing aperoxy bond-containing monomer with one or more monomers selected fromthe group consisting of (meth)acrylic acid esters, unsaturateddicarboxylic acid esters, vinyl esters of fatty acids having 2 to 18carbon atoms and aromatic monomers having 8 to 12 carbon atoms and withone or more monomers selected from the group consisting of tertiarynitrogen-containing monomers and monomers containing either or both of ahydroxyl group and an ether bond, said tertiary nitrogen-containingmonomers being selected from the group consisting of vinylpyrrolidone,vinylpyridine, vinylimidazol, dimethylaminoethyl(meth)acrylate,dimethylaminopropyl(meth)acrylate, dimethylaminoethyl(meth)acrylamide,dimethyl(meth)acrylamide and (meth)acrylonitrile, said hydroxyl or etherbond containing monomers are alkyl(meth)acrylates of the formula:##STR11## wherein R₁₅ is a hydrogen atom or a methyl group, R₁₆ is a C₁-C₆ alkyl group having at least one hydroxyl group or a group of thefollowing formula: ##STR12## wherein R₁₇ and R₁₈ each represent ahydrogen atom or a methyl group, R₁₉ is a hydrogen atom or a C₁ -C₃alkyl group, and n is an integer of 1 to
 60. 3. The viscosity indeximprover according to claim 2, wherein the content of one or moremonomers selected from the group consisting of tertiarynitrogen-containing monomers and monomers containing either or both of ahydroxyl group and an ether bond in the polymer (c) is set to be notmore than 20% by weight.
 4. The viscosity index improver according toclaim 2, wherein the oil-soluble polymer (a), the polymers (b) and (c)are as follows:(a) an oil-soluble polymer which is an ethylene/α-olefincopolymer or styrene/hydrogenated diene; (b) a copolymer to be obtainedby copolymerizing a peroxy bond-containing monomer with (meth)acrylicacid esters; and (c) a copolymer to be obtained by copolymerizing aperoxy bond-containing monomer with (meth)acrylic acid ester and withone or more monomers selected from the group consisting of tertiarynitrogen-containing monomers and monomers containing either or both of ahydroxyl group and an ether bond, said tertiary nitrogen-containingmonomers being selected from the group consisting of vinylpyrrolidone,vinylpyridine, vinylimidazol, dimethylaminoethyl(meth)acrylate,dimethylaminopropyl(meth)acrylate, dimethylaminoethyl(meth)acrylamide,dimethyl(meth)acrylamide and (meth)acrylonitrile, said hydroxyl or etherbond containing monomers are alkyl(meth)acrylates of the formula:##STR13## wherein R₁₅ is a hydrogen atom or a methyl group, R₁₆ is a C₁-C₆ alkyl group having at least one hydroxyl group or a group of thefollowing formula: ##STR14## wherein R₁₇ and R₁₈ each represent ahydrogen atom or a methyl group, R₁₉ is a hydrogen atom or a C₁ -C₃alkyl group, and n is an integer of 1 to
 60. 5. The viscosity indeximprover according to claim 2, wherein the oil-soluble polymer (a) is anethylene/propylene copolymer, a styrene/hydrogenated diene blockcopolymer or a styrene/hydrogenated diene/styrene triblock copolymer. 6.The viscosity index improver according to claim 1, wherein theoil-soluble polymer (a), the polymers (b) and (c) are as follows:(a) oneor more oil-soluble polymers selected from the group consisting ofethylene/α-olefin copolymers, styrene/hydrogenated butadiene orhydrogenated isoprene copolymer, hydrogenated polybutadienes,hydrogenated polyisoprenes and polybutenes; (b) a copolymer to beobtained by copolymerizing a peroxy bond-containing monomer with(meth)acrylic acid esters; and (c) a copolymer to be obtained bycopolymerizing a peroxy bond-containing monomer with meth (acrylic)acidesters and with one or more monomers selected from the group consistingof tertiary nitrogen-containing monomers and monomers containing eitheror both of a hydroxyl group and an ether bond, unsaturated dicarboxylicacid esters, vinyl esters of fatty acids having 1 to 5 carbon atoms andaromatic monomers having 8 to 10 carbon atoms; said tertiarynitrogen-containing monomers being selected from the group consisting ofvinylpyrrolidone, vinylpyridine, vinylimidazol,dimethylaminoethyl(meth)acrylate, dimethylaminopropyl(meth)acrylate,dimethylaminoethyl(meth)acrylamide, dimethyl(meth)acrylamide and(meth)acrylonitrile, said hydroxyl or ether bond containing monomers arealkyl(meth)acrylates of the formula: ##STR15## wherein R₁₅ is a hydrogenatom or a methyl group, R₁₆ is a C₁ -C₆ alkyl group having at least onehydroxyl group or a group of the following formula: ##STR16## whereinR₁₇ and R₁₈ each represent a hydrogen atom or a methyl group, R₁₉ is ahydrogen atom or a C₁ -C₃ alkyl group, and n is an integer of 1 to 60.7. The viscosity index improver according to claim 6, wherein theoil-soluble polymer (a) is an ethylene/propylene copolymer, astyrene/hydrogenated diene diblock copolymer or a styrene/hydrogenateddiene/styrene triblock copolymer.
 8. A process for producing a viscosityindex improver as set forth in claim 1, which comprises heating amixture of the following oil-soluble polymer (a) and the followingperoxy bond-containing copolymer (b) or (c) to such a temperature thatperoxy bonds cleave to effect grafting:(a) one or more oil-solublepolymers selected from the group consisting of ethylene/α-olefincopolymers, styrene/hydrogenated diene copolymers, hydrogenatedpolybutadienes, hydrogenated polyisoprenes, polybutenes,ethylene/(meth)acrylic acid ester copolymers and ethylene/vinyl estercopolymers; and (b) a copolymer to be obtained by copolymerizing aperoxy bond-containing monomer with one or more monomers selected fromthe group consisting of (meth)acrylic acid esters, unsaturateddicarboxylic acid esters, vinyl esters of fatty acids having 2 to 18carbon atoms and aromatic monomers having 8 to 12 carbon atoms; or (c) acopolymer to be obtained by copolymerizing a peroxy bond-containingmonomer with one or more monomers selected from the group consisting of(meth)acrylic acid esters, unsaturated dicarboxylic acid esters, vinylesters of fatty acids having 2 to 18 carbon atoms and aromatic monomershaving 8 to 12 carbon atoms and with one or more monomers selected fromthe group consisting of tertiary nitrogen-containing monomers andmonomers containing either or both of a hydroxyl group and an etherbond, said tertiary nitrogen-containing monomers being selected from thegroup consisting of vinylpyrrolidone, vinylpyridine, vinylimidazol,dimethylaminoethyl(meth)acrylate, dimethylaminopropyl(meth)acrylate,dimethylaminoethyl(meth)acrylamide, dimethyl(meth)acrylamide and(meth)acrylonitrile, said hydroxyl or ether bond containing monomers arealkyl(meth)acrylates of the formula: ##STR17## wherein R₁₅ is a hydrogenatom or a methyl group, R₁₆ is a C₁ -C₆ alkyl group having at least onehydroxyl group or a group of the following formula: ##STR18## whereinR₁₇ and R₁₈ each represent a hydrogen atom or a methyl group, R₁₉ is ahydrogen atom or a C₁ -C₃ alkyl group, and n is an integer of 1 to 60.9. A process for producing a viscosity index improver as set forth inclaim 1, which comprises adding to a lubricant base oil the followingmonomer mixture (d) or (e) and a radical polymerization initiator toeffect copolymerization of the monomer mixture at such a temperaturethat the radical polymerization initiator decomposes and that peroxybonds of the peroxy bond-containing monomer do not substantially cleave,and adding the following oil-soluble polymer (a) to the resultingmixture to be dissolved therein, followed by heating of the mixture tosuch a temperature that peroxy bonds derived from the peroxybond-containing monomer cleave to effect grafting:(a) one or moreoil-soluble polymers selected from the group consisting ofethylene/α-olefin copolymers, hydrogenated styrene/diene copolymers,hydrogenated polybutadienes, hydrogenated polyisoprenes, polybutenes,ethylene/(meth)acrylic acid ester copolymers and ethylene/vinyl estercopolymers; (d) a monomer mixture containing a peroxy bond-containingmonomer and one or more monomers selected from the group consisting of(meth)acrylic acid esters, unsaturated dicarboxylic acid esters, vinylesters of fatty acids having 2 to 18 carbon atoms and aromatic monomershaving 8 to 12 carbon atoms; and (e) a monomer mixture containing aperoxy bond-containing monomer, one or more monomers selected from thegroup consisting of (meth)acrylic acid esters, unsaturated dicarboxylicacid esters, vinyl esters of fatty acids having 2 to 18 carbon atoms andaromatic monomers having 8 to 12 carbon atoms and one or more monomersselected from the group consisting of tertiary nitrogen-containingmonomers and monomers containing either or both of a hydroxyl group andan ether bond, said tertiary nitrogen-containing monomers being selectedfrom the group consisting of vinylpyrrolidone, vinylpyridine,vinylimidazol, dimethylaminoethyl(meth)acrylate,dimethylaminopropyl(meth)acrylate, dimethylaminoethyl(meth)acrylamide,dimethyl(meth)acrylamide and (meth)acrylonitrile, said hydroxyl or etherbond containing monomers are alkyl(meth)acrylates of the formula:##STR19## wherein R₁₅ is a hydrogen atom or a methyl group, R₁₆ is a C₁-C₆ alkyl group having at least one hydroxyl group or a group of thefollowing formula: ##STR20## wherein R₁₇ and R₁₈ each represent ahydrogen atom or a methyl group, R₁₉ is a hydrogen atom or a C₁ -C₃alkyl group, and n is an integer of 1 to
 60. 10. The process forproducing a viscosity index improver as set forth in claim 1, whichcomprises adding to a lubricant base oil the following oil-solublepolymer (a), the following monomer mixture (d) or (e) and a radicalpolymerization initiator to effect copolymerization of the monomermixture at such a temperature that the radical polymerization initiatordecomposes and that peroxy bonds of the peroxy bond-containing monomerdo not substantially cleave, followed by heating of the resultingmixture to such a temperature that peroxy bonds derived from the peroxybond-containing monomer cleave to effect grafting:(a) one or moreoil-soluble polymers selected from the group consisting ofethylene/α-olefin copolymers, styrene/hydrogenated diene copolymers,hydrogenated polybutadienes, hydrogenated polyisoprenes, polybutenes,ethylene/(meth)acrylic acid ester copolymers and ethylene/vinyl estercopolymers; and (d) a monomer mixture containing a peroxybond-containing monomer and one or more monomers selected from the groupconsisting of (meth)acrylic acid esters, unsaturated dicarboxylic acidesters, vinyl esters of fatty acids having 2 to 18 carbon atoms andaromatic monomers having 8 to 12 carbon atoms; and (e) a monomer mixturecontaining a peroxy bond-containing monomer, one or more monomersselected from the group consisting of (meth)acrylic acid esters,unsaturated dicarboxylic acid esters, vinyl esters of fatty acids having2 to 18 carbon atoms and aromatic monomers having 8 to 12 carbon atomsand one or more monomers selected from the group consisting of tertiarynitrogen-containing monomers and monomers containing either or both of ahydroxyl group and an ether bond, said tertiary nitrogen-containingmonomers being selected from the group consisting of vinylpyrrolidone,vinylpyridine, vinylimidazol, dimethylaminoethyl(meth)acrylate,dimethylaminopropyl(meth)acrylate, dimethylaminoethyl(meth)acrylamide,dimethyl(meth)acrylamide and (meth)acrylonitrile, said hydroxyl or etherbond containing monomers are alkyl(meth)acrylates of the formula:##STR21## wherein R₁₅ is a hydrogen atom or a methyl group, R₁₆ is a C₁-C₆ alkyl group having at least one hydroxyl group or a group of thefollowing formula: ##STR22## wherein R₁₇ and R₁₈ each represent ahydrogen atom or a methyl group, R₁₉ is a hydrogen atom or a C₁ -C₃alkyl group, and n is an integer of 1 to
 60. 11. A process for producinga viscosity index improver as set forth in claim 1, which comprises afirst step of adding to an aqueous suspension of particles of thefollowing oil-soluble polymer (a) the following monomer mixture (d) or(e) and a radical polymerization initiator, and heating the resultingmixture at such a temperature that the radical polymerization initiatordoes not substantially decompose to impregnate the particulateoil-soluble polymer (a) with the monomer mixture and the radicalpolymerization initiator, followed by heating of the resulting aqueoussuspension at such a temperature that the radical polymerizationinitiator decomposes and that the peroxy bonds of the peroxybond-containing monomer do not substantially cleave to effectcopolymerization of the monomer mixture in the particulate oil-solublepolymer (a) and obtain a graft precursor; and a second step ofmelt-kneading the graft precursor at such a temperature that peroxybonds derived from the peroxy bond-containing monomer cleave to effectgrafting:(a) one or more oil-soluble polymers selected from the groupconsisting of ethylene/α-olefin copolymers, styrene/hydrogenated dienecopolymers, hydrogenated polybutadienes, hydrogenated polyisoprenes,polybutenes, ethylene/(meth)acrylic acid ester copolymers andethylene/vinyl ester copolymers; and (d) a monomer mixture containing aperoxy bond-containing monomer and one or more monomers selected fromthe group consisting of (meth)acrylic acid esters, unsaturateddicarboxylic acid esters, vinyl esters of fatty acids having 2 to 18carbon atoms and aromatic monomers having 8 to 12 carbon atoms; or (e) amonomer mixture containing a peroxy bond-containing monomer, one or moremonomers selected from the group consisting of (meth)acrylic acidesters, unsaturated discarboxylic acid esters, vinyl esters of fattyacids having 2 to 18 carbon atoms and aromatic monomers having 8 to 12carbon atoms and one or more monomers selected from the group consistingof tertiary nitrogen-containing monomers and monomers containing eitheror both of a hydroxyl group and an ether bond, said tertiarynitrogen-containing monomers being selected from the group consisting ofvinylpyrrolidone, vinylpyridine, vinylimidazol,dimethylaminoethyl(meth)acrylate, dimethylaminopropyl(meth)acrylate,dimethylaminoethyl(meth)acrylamide, dimethyl(meth)acrylamide and(meth)acrylonitrile, said hydroxyl or ether bond containing monomers arealkyl(meth)acrylates of the formula: ##STR23## wherein R₁₅ is a hydrogenatom or a methyl group, R₁₆ is a C₁ -C₆ alkyl group having at least onehydroxyl group or a group of the following formula: ##STR24## whereinR₁₇ and R₁₈ each represent a hydrogen atom or a methyl group, R₁₉ is ahydrogen atom or a C₁ -C₃ alkyl group, and n is an integer of 1 to 60.12. A lubricating oil composition comprising a lubricating oilcontaining the viscosity index improver according to claim
 1. 13. Aviscosity index improver comprising a graft copolymer (i), polymer (ii)or copolymer (iii), or polymer (ii) and coploymer (iii):graft copolymer(i) is obtained by grafting one or more oil-soluble polymers selectedfrom the group consisting of ethylene/α-olefin copolymers,styrene/hydrogenated diene copolymers, hydrogenated polybutadienes,hydrogenated polyisoprenes, polybutenes, ethylene/(meth)acrylic acidester copolymers and ethylene/vinyl ester copolymers to or with peroxybond-containing copolymers (b) to be obtained by copolymerizing a peroxybond-containing monomer with one or more monomers selected from thegroup consisting of (meth)acrylic acid esters, unsaturated dicarboxylicacid esters, vinyl esters of fatty acids having 2 to 18 carbon atoms andaromatic monomers having 8 to 12 carbon atoms; ora copolymer (c) to beobtained by copolymerizing a peroxy bond-containing monomer with one ormore monomers selected from the group consisting of (meth)acrylic acidesters, unsaturated dicarboxylic acid esters, vinyl esters of fattyacids having 2 to 18 carbon atoms and aromatic monomers having 8 to 12carbon atoms and with one or more monomers selected from the groupconsisting of tertiary nitrogen-containing monomers and monomerscontaining either or both of a hydroxyl group and an ether bond; graftpolymer (ii) is one or more oil-soluble polymers selected from the groupconsisting of ethylene/α-olefin copolymers, styrene/hydrogenated dienecopolymers, hydrogenated polybutadienes, hydrogenated polyisoprenes,polybutenes, ethylene/(meth)acrylic acid ester copolymers andethylene/vinyl ester copolymers; graft copolymer (iii) is a copolymerderived from a copolymer to be obtained by copolymerizing a peroxybond-containing monomer with one or more monomers selected from thegroup consisting of (meth)acrylic acid esters, unsaturated dicarboxylicacid esters, vinyl esters of fatty acids having 2 to 18 carbon atoms andaromatic monomers having 8 to 12 carbon atoms; or a copolymer (c) to beobtained by copolymerizing a peroxy bond-containing monomer with one ormore monomers selected from the group consisting of (meth)acrylic acidesters, unsaturated dicarboxylic acid esters, vinyl esters of fattyacids having 2 to 18 carbon atoms and aromatic monomers having 8 to 12carbon atoms and with one or more monomers selected from the groupconsisting of tertiary nitrogen-containing monomers and monomerscontaining either or both of a hydroxyl group and an ether bond, saidtertiary nitrogen-containing monomers being selected from the groupconsisting of vinylpyrrolidone, vinylpyridine, vinylimidazol,dimethylaminoethyl(meth)acrylate, dimethylaminopropyl(meth)acrylate,dimethylaminoethyl(meth)acrylamide, dimethyl(meth)acrylamide and(meth)acrylonitrile, said hydroxyl or ether bond containing monomers arealkyl(meth)acrylates of the formula: ##STR25## wherein R₁₅ is a hydrogenatom or a methyl group, R₁₆ is a C₁ -C₆ alkyl group having at least onehydroxyl group or a group of the following formula: ##STR26## whereinR₁₇ and R₁₈ each represent a hydrogen atom or a methyl group, R₁₉ is ahydrogen atom or a C₁ -C₃ alkyl group, and n is an integer of 1 to 60.14. The viscosity index improver according to claim 13, wherein thecontent of one or more monomers selected from the group consisting oftertiary nitrogen-containing monomers and monomers containing either orboth of a hydroxyl group and an ether bond in the polymer (c) is set tobe not more than 20% by weight.
 15. A lubricating oil compositioncomprising a lubricating oil containing the viscosity index improveraccording to claim
 14. 16. The viscosity index improver according toclaim 13, wherein the oil-soluble polymer (a) is an ethylene/propylenecopolymer, a styrene/hydrogenated diene diblock copolymer or astyrene/hydrogenated diene/styrene triblock copolymer.